The present invention relates in general to an automatic test system or apparatus and pertains, more particularly, to a novel automatic fixturing system for printed circuit board testing.
It is common practice in the electronics industry to subject printed circuit boards and their assemblies to various types of electrical tests to ensure quality and reliability. These tests include both the continuity testing of bare (unloaded) boards to detect unwanted open circuits and short circuits, and loaded board testing of completed printed circuit board assemblies to ensure the correct functioning of electronic components loaded on the board. In either case, testing is accomplished by means of automatic test equipment (ATE) which generally comprises means for applying suitable electrical stimuli (inputs) to appropriate test points on the printed circuit board and for measuring the resulting signals (outputs) from other selected test points. In particular, electrical connections between the selected test ponts on the printed circuit board and the input/output terminals of the ATE system are accomplished by means of ATE fixtures or fixturing systems.
Typically, an ATE fixture incorporates a `bed of nails` comprising a parallel array of spring-loaded test probes carried in probe receptacles which are secured in a probe plate in a geometrical pattern precisely matching, in distribution and number, the selected test point sites on the printed circuit board. The lower extremities of the probe receptacles are wired individually to a suitable interface connector by which means electrical connection is made between the individual probes and the input/output terminals of the appropriate ATE system. Printed circuit boards to be tested are inserted individually in the fixture with the test points carefully aligned with the test probe array. The printed circuit board is then forced against the test probe array by mechanical, hydraulic, pneumatic or vacuum-operated means so that the spring-loaded probes are compressed and suitable electrical connection is made between each test point and the head of the corresponding probe, which typically incorporates a sharpened tip or multiple points to penetrate the solder run at the test point location. Transmission of stimuli and resultant signals between the ATE system and the printed circuit board under test is thus made possible by the fixture.
Because of the generally unique nature of the test-point pattern on a given printed circuit board type, a new and different fixture is usually required for each and every different printed circuit board type tested. Since a typical user of ATE equipment may have to test many types of circuit boards each year, conventional fixtures of the type described above have three serious disadvantages, namely:
(1) Cost--a typical fixture can cost from several thousand to several tens of thousands of dollars and has little or no scrap value once testing of its printed circuit board type is completed; PA0 (2) Time to Fabricate--typical delivery times range from 2 to 6 weeks which can often result in delay of production schedules; PA0 (3) Storage Requirements--the typical conventional fixture is bulky and requires considerable storage space when not in use; PA0 (4) Handling Requirements--the bulk and weight of the typical fixture poses significant handling problems for test personnel. PA0 (a) Since all the test probes in the base matrix must be compressed regardless of the number of `active` probes actually required in a given case, very large forces are required to operate the fixture. As a result, the fixture structure must be very strong (and hence bulky) and, in particular, vacuum-operation (which is generally the simplest and least expensive option) is ruled out; PA0 (b) A large-capacity ATE system must be dedicated to service the fixture (typically, 100 input/output terminals must be dedicated to each square inch of test area in the fixture). This requirement specifically eliminates the potential application of the fixture to loaded board testing for which ATE system capacity is generally extremely limited; PA0 (c) By definition, the fixture cannot be used to test boards on which all of the test points do not lie on a grid matrix of standard dimensions; PA0 (d) While the cost of dedicated hardware (i.e. hardware peculiar to each different type of printed circuit board) is essentially eliminated, the capital cost of the fixturing system is substantial by virtue of the size and strength of its structure and the cost of the large number of test probes which must be permanently installed in it, regardless of the quantity actually involved in a specific test. Moreover, substantial additional cost is involved in the large-capacity ATE system which must necessarily be dedicated to the fixture (see (b) above). PA0 (a) Manual assembly of the offset probe head is time-consuming, particularly in the case of large fixtures incorporating several thousand probes; PA0 (b) Each time an offset probe for a specific printed circuit board is broken down (in order to re-use the offset probes in a different test) and subsequently re-assembled the offset probes will generally acquire new (different) locations in the lower base matrix, due to the somewhat arbitrary nature of the insertion process. Consequently, the test points will be interconnected to a different set of ATE input/output terminals so that the ATE system must be re-calibrated in order to identify faults detected during testing with specific localities on the printed circuit boards under test. PA0 (1) As noted earlier, they require a very large ATE system capacity (typically, 100 input/output terminals per square inch of test area) which is generally not available on loaded board test systems; PA0 (2) Unlike the simple continuity testing required for bare (unloaded) boards, loaded board testing involves the use of several different classes of input/output lines (e.g. power busses, analog and digital signal lines of various types, ground lines, etc.) Clearly, none of the universal fixturing systems outlined above provides any means for differentiating between these various types of test requirements and, hence, for ensuring the correct interfacing for each test point with the appropriate type of input/output terminal on the ATE system.
The foregoing problems associated with conventional fixturing have long been recognized in the industry and have spurred efforts to develop a universal fixturing system that would accommodate all, or at least a very wide range of printed circuit board types with virtually no requirement for individual dedicated hardware for each separate printed circuit board type. Several different types of universal of quasi-universal fixtures hve indeed been developed as described briefly below.
One fixture is a universal grid matrix. This system relies on the fact that many printed circuit boards are laid out so that all component holes (which accommodate component leads) and hence all potential test points lie on a regular grid matrix (typically an 0.01" grid). For the class of such boards, fixturing can obviously be effected by means of a corresponding grid matrix array of test probes (typically incorporating 100 test probes per square inch on an 0.01" grid spacing) secured in a probe plate and permanently wired to the ATE system via the fixture interface connector. Hardware or software masking is generally provided to insulate or eliminate from consideration those probes which do not coincide with test points on the particular printed circuit board under test.
Universal fixtures of this type suffer generally from four major drawbacks as follows:
Another type of fixturing system may be referred to as a universal grid with electrical/mechanical probe activation. This device is a development of the basic grid matrix above in which (typically) a special programming card (specific for each different printed circuit board type) is used to mechanically and electrically activate those and only those test probes within the total grid array, which are actually required for the test in progress. While this eliminates the need to compress the entire array (problem (a) in (1) above) the need for the relatively expensive dedicated programming cards degrades the original universality of the system.
Another system is referred to as a universal grid matrix with offset probe head. The concept of the offset probe was introduced essentially to expand the application of the universal fixture to printed circuit boards on which all of the test points do not lie on a standard grid. The offest probe is typically designed so that it can be bent, flexed or inclined away from its original axis to provide a small amount of lateral offset (typically up to 0.07") between its upper extremity (tip) and its lower extremity (base) when inserted in a special offset probe head. The offset probe head, which must be specially designed for a specific printed circuit board, incorporates an array of such probes so distributed and inclined, bent or flexed that their lower, (base) extremities conform to the universal grid matrix while their upper, (tip) extremities conform precisely with the desired pattern of test points on the printed circuit board. Thus, when the offset probe head is suitably inserted between the universal grid matrix and the printed circuit board under test, electrical continuity is established between the test points and the ATE system via the offset probes and their corresponding counterparts within the basic grid matrix.
In order to minimize the recurring dedicated cost re-introduced by the offset probe head, this head is typically constructed of two low-cost plastic plates. The upper probe plate is drilled in the required test point pattern, while the lower (base) plate is drilled in a grid pattern corresponding to the universal grid matrix. In the most common embodiment, straight probes are manually inserted in groups through the test holes in the upper plate and allowed to drop so that their lower extremities become located in laterally-adjacent base holes in the lower plate.
The offset probe head concept extends the application of the universal grid fixture to printed circuit boards whose test points do not conform to any standard grid matrix. The concept suffers from several disadvantages besides the (albeit low) cost of the offset probe heads.
Still another fixture system is referred to as a universal base matrix with offset probes. This type of fixture utilizes the offset probe head of the previous system but dispenses with the original universal grid matrix of probes. In this case, the offset probes within the head interface directly with a grid matrix of fixed contacts which are wired directly to the interface connector. Thus, instead of being an optional addendum to the universal grid fixture to extend its application to printed circuit boards with off-grid test points, the offset probe head, in this case, is an integral part of the fixture and is used in testing a types of printed circuit board, including those which are laid out on a universal grid. In prctice, it provides similar advantages and suffers similar drawbacks to those indicated for the previous system.
Apart from the specific individual disadvantages of each type of the four universal fixturing systems described above, it should be noted that all of these systems are generally unsuitable for loaded board testing for two reasons, namely: